1. Technical Field
The invention relates generally to ion implantation, and more particularly, to architecture for a ribbon ion beam ion implanter system.
2. Background Art
Ion implantation is a standard technique for introducing conductivity altering impurities into, or doping, semiconductor wafers. A typical ion implantation process uses an energetic ion beam to introduce impurities into work pieces, i.e., semiconductor wafers. As is well known, introducing the impurities at a uniform depth and dose into the wafers is important to ensure that semiconductor devices being formed operate properly.
FIG. 1 shows schematically, in three dimensions, a conventional implantation of ions into a wafer. X-Axis and Y-Axis constitute a transverse ion beam scan plane. An ion beam is delivered (desirably) parallel to the Z-Axis and strikes the planar surface of the wafer. The X-Axis is horizontally perpendicular to the Z-Axis. In a ribbon ion beam system, the ion beam is a ribbon along the X-Axis. The Y-Axis is vertically perpendicular to the ion beam plane (i.e., the XZ-coordinate plane). The wafer is scanned up and down along another scan path parallel to the Y-Axis by moving the wafer up and down.
Transport of a low energy (high current) ion beam through an ion implanter system is difficult due to the large defocusing effect of the ion beam's space-charge. This can be alleviated by extracting, and mass-analyzing the beam at a higher energy (e.g., >approximately 10 kV), and then decelerating the beam to a final energy, which can be as low as approximately 200 eV when the ion beam is close to the wafer. However, any ions that get neutralized just before and during deceleration, will not be decelerated fully, and will impinge on the wafer at the higher energy, resulting in harmful energy contamination.
For systems that involve the mass analysis of a ribbon ion beam by dispersion in the plane of the ribbon, resulting in a fanned ribbon ion beam, it is also necessary to parallelize the ion beam, which is currently performed by a sector electromagnet. Controlling the parallelism or angle of an ion beam is important for the proper operation of various different types of devices and processes. The depth at which impurities are implanted depends in part upon the parallelism of an ion beam along a desired direction, typically perpendicular, to the crystal structure of the semiconductor. Therefore, it is important to control the angle of the ion beam during implantation to maintain a desired parallelism (i.e., desired direction) of the ion trajectories relative to a wafer's crystal structure. In particular, in order to achieve repeatable implant results, the angle of the ion beam should be known and controlled to a range of error of less than 1° from parallel to the desired direction, especially for high energy implants and channeled implants. In terms of a fanned ribbon ion beam, if the parallelizing is completed after the deceleration lens, then transport length is added to the system, which further impairs the delivery of the low energy beam to the wafer. Note that such a sector parallelizing lens is quite large, and can add as much as 1 m to the beam transport after deceleration.
In view of the foregoing, there is a need in the art for a way to provide acceleration/deceleration and parallelizing of a ribbon ion beam without adding length to an ion implanter system.